With increasing penetration of wind power generation, the requirements for the connection of Wind Power Plants (WPP) to the electrical grid are defined by new and emerging grid connection codes. The grid connection requirements vary in different parts of the world, but they share common aims, like to permit the development, maintenance and operation of a coordinated, reliable and economical transmission or distribution system. Wind power plants differ from other traditional generation sources; thus, they are particular in certain aspects of their control and layout. Therefore, replacing traditional power plants, including their control characteristics, during periods of strong wind could be a concern. Grid operators are solving this challenge by means of redacting specific sections in the grid codes for WPP performance.
The new requirements generally demand that wind power plants provide ancillary services to support the network in which they are connected. In the present case, voltage regulation is of interest, and has been introduced recently by some grid operators. Voltage regulation with wind power plants entails the integration of all the available control devices installed at substations and plant.
Normally, the requirements of the utilities can be solved by the turbines and the park controller. When the wind turbines are not enough to fulfill the grid code requirements, then reactive power compensation equipment should be installed. This equipment could be static compensators if the plant needs more voltage dynamic capability, mechanical switching capacitors/reactors if the requirement is related to steady state performance, or just simply it could be a combination of both. The adoption of each one of the possible solutions, and choosing the method and combination of components for voltage regulation is one of the design decisions. The prevalent position is minimizing the cost of investments while offering the fulfillment of the requirements.
WPPs are composed by a large number of generation units, thus introducing communication delays in the plant control when processing the data and these could be in the range of hundreds of milliseconds.
The reactive power and voltage control of the wind turbines is relatively fast, with characteristic time constants in tens of milliseconds, the reactive power and voltage control of the central plant control is somewhat slower, with characteristic time constants in a range of hundreds of milliseconds, and this is used for voltage control in the agreed node in the transmission system in accordance with the requirements of the grid codes. As voltage control is actuated by reactive power from the turbines, it is possible to affirm that the voltage control design will cover an inner reactive power/voltage control loop, located at turbine level, and an outer voltage control loop, located at the Point of Common Coupling (PCC) where it is intended to control the voltage.
It may be seen as an object of embodiments of the present invention to provide a method and a control arrangement for controlling a reactive power source.